1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking a plurality of unit fuel cells, in particular, a fuel cell stack having a drainage structure for draining produced water, or condensed water accompanied with the electric power generation.
Priority is claimed on Japanese Patent Application No. 2005-331095, filed Nov. 16, 2005, the content of which is incorporated herein by reference.
2. Description of the Related Art
In a known fuel cell, an anode and a cathode are provided on either side of a solid polymer electrolyte membrane so as to form a membrane electrode assembly. The membrane electrode assembly is placed between a pair of separators for forming reaction gas passages, thereby forming a unit fuel cell (called a “unit cell” below). Generally, a specific number of unit cells are stacked, and the stacked body is supported between end holding members, thereby forming a fuel cell stack.
In each unit cell, hydrogen gas and oxygen gas (actually, air which includes oxygen) are respectively supplied as reaction gases to the anode and the cathode, so that an electrochemical reaction between oxygen and hydrogen ions which pass through the electrolyte membrane, generates electric power. In the electrochemical reaction, in addition to the electric power generation, reaction water is generated at the cathode, and this reaction water is drained and discharged via the cathode to the outside of the fuel cell stack, together with an exhausted gas (called an “off gas” below). Part of the reaction water generated at the cathode is back diffused toward the anode through the electrolyte membrane, and this back-diffused reaction water is drained and discharged via the anode to the outside of the fuel cell stack, together with the anode off gas.
To each unit cell, reaction gases including water vapor are supplied so as to humidify the electrolyte membrane. The water vapor in the reaction gases is likely to condense in the unit cell or gas discharge passages, and transformed into condensed water, which is also drained to the outside of the fuel cell stack, together with the anode off gas.
In this type of fuel cell stack, gas supply passages and gas discharge passages, which communicate with reaction gas passages of each unit cell, are formed in a manner such that they penetrate through every unit cell and one of the end holding members. An end of each gas discharge passage, which is formed at the one of the end holding members, is arranged at a lower position of the fuel cell stack so that the above-described reaction water or condensed water (called “residual water” below) can be reliably drained to the outside.
However, an external piping member having a relatively large diameter is connected to the end of the gas discharge passage at said one of the end holding members; thus, it may be difficult to arrange the end of the gas discharge passage at a sufficiently low position of the fuel cell stack.
As a technique for solving the above problem, Japanese Unexamined Patent Application, First Publication No. 2000-164237 discloses a fuel cell stack having an end holding member which has (i) a gas passage bending upward from a gas discharge passage of the unit cells, and (ii) a branch passage for drainage, which communicates with an inner and lower end part of the gas discharge passage of the unit cells.
However, in this conventional fuel cell stack, the branch passage for drainage is provided only at a lower end position of one of the end holding members. Therefore, when the fuel cell stack is inclined in a manner such that the other end holding member is positioned lower, it is difficult to drain the residual water from the inside of the fuel cell stack.
In order to solve this problem, an improved structure has been examined in which similar branch passages for drainage are provided at both end holding members (positioned at front and back sides), so that the residual water can be drained through either branch passage.
However, generally, it is preferable to perform drainage of the fuel cell stack through a single piping member, in consideration of marketability. Therefore, when the branch passages for drainage are provided at both end holding members, as described above, additional piping should be provided outside the fuel cell stack, so as to collect waterdrops drained from both branch passages. Accordingly, the above presently-examined fuel cell stack needs long external piping, which may considerably increase the manufacturing cost. In addition, the space occupied by the piping should be increased.